The potential repair of articular cartilage in human using porous scaffolds has been described. Mears in U.S. Pat. No. 4,553,272 describes a method for osteochondral repair focusing on the use of starter cells, specific pore sizes in a porous scaffold, and providing a barrier between the two pore sizes. There is no mention of the use of biodegradable scaffolds or the necessity of providing a scaffold that can withstand physiological loads. Hunziker in U.S. Pat. No. 5,206,023 teaches a method for articular cartilage repair involving pretreating the defect area with enzymes to remove proteoglycans, then providing a biodegradable carrier (scaffold) to provide proliferation agents, growth factors, and chemotactic agents.
Vert et al. in U.S. Pat. No. 4,279,249 describe a solid biodegradable osteosynthesis device made of a fiber-reinforced composite. The fibrous component is a biodegradable polymer high in glycolide content and the matrix component is high in lactide units. There is no mention of porous devices and the favorable mechanical properties achieved through fiber reinforcement are obtained through typical stacking and layering techniques known in the art. Uniform distribution of fibers throughout the matrix is not disclosed.
The prior art does not appear to teach how to optimize mechanical properties by reinforcing highly porous materials (50%-90% porous). Nijenhuis et al. in Eur.0,277,678 describe a biodegradable, porous scaffold preferably incorporating biodegradable reinforcing fibers. The scaffold has a bi-porous structure (bimodal pore distribution) made using a combination of solution-precipitation and salt-leaching techniques. Although the fibers are incorporated to xe2x80x9creinforcexe2x80x9d the scaffold, no evidence is presented to verify that the mechanical properties are actually improved through such reinforcement and the fibers appear to be randomly aligned.
Stone et al. in U.S. Pat. No. 5,306,311 describe a prosthetic, resorbable articular cartilage composed of a dry, porous, volume matrix of randomly or radially oriented, allegedly biocompatible and bioresorbable fibers. Stone""s patent speaks mainly of natural polymeric fibers, such as collagen and elastin, which are harvested and purified from xenogenic sources. The fibers are then cross-linked to provide a cohesive scaffold. The ability of the scaffold to support articulating joint forces is not shown.
All publications and patent applications referred to herein are fully incorporated by reference to the extent not inconsistent herewith.
This invention provides a fiber-reinforced, polymeric implant material useful for tissue engineering, and method of making same. The implant material preferably comprises a polymeric matrix, preferably a biodegradable matrix, having fibers substantially uniformly distributed therein in predominantly parallel orientation as shown in FIGS. 1 and 2. In preferred embodiments, porous tissue scaffolds are provided which facilitate regeneration of load-bearing tissues such as articular cartilage and bone.
The material of this invention is used to prepare porous, fiber-reinforced, biodegradable tissue scaffolds whose fibrous supports are oriented predominantly in a single direction. The scaffold may be implanted into humans or animals to provide support for physiological loads applied parallel to the predominant direction of orientation of the fibers. For example, in an osteochondral site on the femoral condyle, the primary direction of loading is perpendicular to the surface of the cartilage. The oriented fibers act like struts in a bridge support to provide strength and stiffness to the pore walls of the scaffold and provide a characteristic columnar pore architecture especially suitable for cell in growth. The orientation of the fibers also causes the mechanical properties of the scaffold to be anisotropic, i.e., the higher strengths provided by the fibers is maximal in the direction parallel to the fibers, thus providing primary support for physiological loads where they are highest.
Orientation of the fibers is achieved through a dissolution-precipitation process combined with a novel kneading and rolling process. Using various amounts of fiber reinforcement, the mechanical properties of the scaffold may be tailored to the host tissue environment for optimal performance.
Alternatively, the polymer used may be non-biodegradable, and/or the implant may be made non-porous (fully dense) for use as a permanent implant, such as a bone plate in locations requiring load bearing.
The fibers are substantially uniformly distributed throughout the polymer matrix, that is the number of fibers present in a selected portion of the matrix which is large enough to contain several macropores should be substantially the same as (within at least about 20% of) the number of fibers present in any other such selected portion of the matrix. xe2x80x9cMacroporesxe2x80x9d are the larger, columnar-shaped voids formed in the process of manufacturing the material as shown in FIGS. 1 and 2.
The invention may be used for a variety of tissue engineering applications, including osteochondral defect repair, partial and full thickness cartilage defect repair, bone graft substitute, bone graft onlay, ligament or tendon augmentation, oral/maxillofacial surgery, and other reconstructive surgery. The invention is particularly useful for, but not limited to, applications where the implant is to be placed in a defect in a load-bearing tissue, i.e., where stresses applied to the implant once placed in the defect are high in one direction compared to relatively perpendicular directions. One example is in an osteochondral or full thickness cartilage defect, where during such activities as normal walking, there are very high compressive stresses perpendicular to the surface of the cartilage, whereas the stresses parallel to the surface are much less. Another example is alveolar ridge augmentation, where primarily one-directional compressive stresses are due to biting or chewing.
The reinforcing fibers may be made of any suitable biodegradable material by methods known to the art or may be commercially available fibers. Polyglycolide (PGA) fibers are currently available from several sources including Albany International, Sherwood Davis and Geck and Genzyme Surgical Products. Fibers from sutures may also be used, e.g., Vicryl(copyright) (90:10 poly [glycolide:lactide]) from Ethicon (Johnson and Johnson). They are preferably synthetic fibers, and are preferably of a length short enough not to interfere with processability, e.g., less than about 1 cm. They can be chopped to the desired length from longer fibers, preferably they have a length between about 0.5 mm and about 1.0 cm and more preferably between about 0.5 mm and about 4.5 mm. The fibers preferably have a diameter between about 5 xcexcm and about 50 xcexcm, more preferably between about 5 xcexcm and about 25 xcexcm.
The reinforcing fibers preferably have mechanical properties that are not substantially compromised when tested in a physiological (aqueous, 37xc2x0 C.) environment. Any biocompatible material can be used to make the fibers. The fibers are preferably insoluble in the solvent used to dissolve the matrix polymer. For articular cartilage repair, the fibers are preferably made from polyglycolide (PGA) or a glycolide-lactide copolymer with a glycolide content above 80%. For bone repair, the fibers may be made of a biodegradable glass such as calcium phosphate orbioactive ceramic. The volume fraction of fibers within the composite scaffold is preferably between about 5% and about 50%, and more preferably between about 10% and about 30%.
The reinforcing fibers used in this invention may alternatively be hollow fibers known to the art. The hollow fibers provide channels to aid in cell and tissue infiltration and can additionally be filled with bioactive agent for delivery to the tissue.
Biodegradable polymers or other biodegradable materials known to the art may be used for the biodegradable matrix. Some examples of suitable biodegradable polymers are alpha-polyhydroxy acids, polyglycolide (PGA), poly(L-lactide), poly(D,L-lactide), poly(xcex5-caprolactone), poly(trimethylene carbonate), poly(ethylene oxide) (PEO), poly(xcex2hydroxybutyrate) (PHB), poly(xcex2-hydroxyvalerate) (PHVA), poly(p-dioxanone) (PDS), poly(ortho esters), tyrosine-derived polycarbonates, polypeptides and copolymers of the above.
The fibers in the implant material of this invention are preferably oriented predominantly parallel to each other, meaning that greater than fifty percent of the total length of the totality of the fibers, and preferably greater than 75%, are oriented in the same direction or within about 20 degrees, more preferably within about 15 degrees, of the same direction. Preferably, at least as great a portion of the total length of the fibers are oriented as close to parallel to each other as depicted in FIG. 1, a scanning electron micrograph showing a material of this invention.
The material of this invention is preferably porous. The pores are preferably interconnected to allow cell migration and extracellular matrix continuity. Interconnectivity here is defined as substantial physical continuity of porous space throughout the scaffold. The presence of fibers during the foaming stage of the manufacturing process helps to insure a minimum of closed-cell pores, which in turn, maximizes the number of open cells, a measure of the interconnectivity. It is preferred that pore distribution and size be substantially uniform. FIGS. 7A and 7B graph pore size distribution in implant materials without fibers in FIG. 7A and with fibers in FIG. 7B. Significant improvement in uniformity of pore distribution is shown in FIG. 7B with its narrow distribution peak compared to that of FIG. 7A. Uniformity of pore distribution in a fiber-reinforced material of this invention is defined herein as giving rise to a distribution curve showing significantly more uniformity than the distribution curve of the same material without fibers. Fibers help to insure that pore size is uniform by providing well-distributed nucleation sites during the foaming process. It is preferred that the pores have average linear dimensions (distance between pore walls, also referred to herein as xe2x80x9cdiameterxe2x80x9d) large enough to accommodate ingrowing cells, e.g., at least about 25 xcexcm, and less than about 300 xcexcm, more preferably between about 50 and about 250 xcexcm.
The porosity (pore volume) of the scaffold is preferably between about 50% and about 80%, and more preferably between about 60% and about 70%. Ideally, the scaffold should be sufficiently porous to facilitate tissue regeneration but not so porous as to compromise its mechanical integrity. The oriented fiber-reinforced material has a characteristic columnar architecture which is xe2x80x9cbiomimeticxe2x80x9d of the columnar cell orientation of chondrocytes in articular cartilage.
The porous material of this invention may be used as a tissue scaffold in vivo or in vitro. That is, the material acts as a scaffold (framework) providing support and spaces for in growth of cells either after it has been placed within a tissue defect in the patient""s body, or alternatively, the scaffold material of this invention may be preseeded with autologous or allogenic cells or cell-containing media before implantation. By adding cells to the scaffold ex vivo before implantation, the formation of the desired tissue or organ type can be accelerated. For example, addition of bone marrow to the implant accelerates the formation of bone throughout the scaffold due the presence of osteoprogenitor and angiogenic cells. Addition of hepatocytes to a scaffold material has been reported to form liver tissue; similarly, addition of chondrocytes has been reported to form cartilage.
Cells can be pre-treated with growth and differentiation factors to induce proliferation or a desired phenotype.
The implants of this invention may be used to deliver bioactive agents such as growth factors, antibiotics, hormones, steroids, anti-inflammatory agents and anesthetics in timed manner, such as in a timed burst or a controlled-release pattern.
The implant material of this invention may be used as one phase of a multiphase implant, e.g. as described in U.S. Pat. No. 5,607,474, incorporated herein by reference to the extent not inconsistent herewith. Preferably, the implant material of this invention has mechanical properties similar or identical to those of the tissue into which the implant is to be placed, controlled by the amount and type of fibers used. The effect of fiber content on mechanical properties is shown in FIGS. 3-6.
Methods of making implant materials are also provided herein. A method for making a fiber-reinforced, porous, biodegradable tissue scaffold implant material comprising fibers aligned predominantly in one direction comprises:
a) dissolving a biodegradable polymer in a suitable organic solvent to form a solution;
b) dispersing the fibers in a suitable non-solvent for the polymer to form a suspension.
c) precipitating the polymer mixed with fibers as a coherent mass from solution by mixing the suspension and solution;
d) kneading and rolling the coherent mass of fibers and polymer to orient the fibers predominantly parallel to each other; and
e) applying heat and vacuum pressure to the mass to foam and cure it.
Methods for the preparation of precipitated polymers are well-known to the art. In general, the process comprises mixing a dried polymer mix with a solvent, preferably acetone, precipitating the polymer mass from solution with a non-solvent, e.g. ethanol, methanol, ether or water, extracting solvent and precipitating agent from the mass until it is a coherent mass which can be pressed into a mold or extruded into a mold, and curing the composition to the desired shape and stiffness. Kneading and rolling may be performed as described in PCT Publication WO 97/13533, incorporated herein by reference to the extent not inconsistent herewith. The kneading may be done by hand or machine and should be continued until the fibers are predominantly aligned substantially parallel to each other, e.g. until the mass becomes difficult to work. Kneading and rolling should be stopped just short of the point where the mass becomes too hard to press into the mold. If it is desired to align the fibers in a single plane, the mass should be rolled out for placement in a flat, shallow mold. If it is additionally desired to align the fibers in a single direction, the mass should additionally be stretched in the desired direction of alignment. Placement in a cylindrical mold is preferred when fiber alignment in a single direction is desired, with molds having a length to diameter ratio of about 1 to 10 being preferred. Increasing degrees of parallel fiber orientation can be achieved (up to a limit) by increasing the length to diameter ratio. Curing and foaming the polymer in the mold to form a porous implant may then be done.
After the fibers have been dispersed in the porous scaffold, the scaffold may optionally be pressed, e.g., compression molded, preferably at elevated temperatures until all the pores have been collapsed into a fully dense (non-porous) composite. This approach is especially effective in creating a fiber reinforced composite in which fibers are evenly dispersed and results in good interfacial bonding between the fibers and the matrix.
The temperatures required for pressing are as known to the art, those at which the material softens sufficiently to allow intermingling of the polymer chains and collapse of the pores under the pressure being applied. The time and temperature used for compression molding should be sufficient to ensure that no significant residual stresses are present (i.e., the material does not expand when the pressure is released. Using an amorphous PGA (75:25), for example, a temperature of about 100xc2x0 C. is used. For semi-crystallizing polymers such as L-PLA, a temperature of at least about 180xc2x0 C. is required.